The combustion apparatus of a gas turbine engine is required to operate over a wide range of conditions. It is important that throughout this range of operating conditions, the fuel and air mixture which is directed into the apparatus is as thoroughly mixed as possible. If such thorough mixing is not achieved, then following combustion of the mixture, zones of combustion products will appear which are hotter than the remainder of the combustion products. This gives rise to variations in the temperature distribution of the combustion products as they exit the combustion apparatus. As a direct consequence of this, the nozzle guide vanes and other parts of the turbine which normally lie downstream of the combustion apparatus are subjected to localised overheating.
Conventionally, the parts of the turbine which lie downstream of the combustion apparatus are provided with internal cooling air passages in order to ensure that they do not overheat. However, in order to cope with the localised overheating which can result from poor fuel and air mixing, the cooling air flow to the turbine parts is higher than would otherwise be the case. This in turn has a prejudicial effect upon overall engine efficiency. A further problem associated with such localised overheating is that it can have a detrimental effect upon turbine component life.